Optical metrology systems are used for inspecting micron, and submicron level details of semiconductor wafers. The present invention relates to such a system and provides for determining the position of components of the system relative to other components of the system, so that the precise location of sites being inspected on the wafer can be determined.
Optical metrology systems are used in the semiconductor industry to image and measure properties of semiconductor wafers. Typically, the wafers are supported on a movable stage and scanned with respect to an optical probing beam. In the past, the most commonly used type of system had controls which allowed a user to move the stage in both the X and Y directions. These X-Y stages could also be combined with a rotational mounting. The optical metrology systems are typically provided with optics and imaging systems so the operator can view the wafer.
In order to reduce the footprint of the optical metrology system R-theta stages, also referred to as polar coordinate stages, were developed which can move the wafer along one linear axis (R) and rotate the wafer about a center of rotation. Such a stage can be used to align any portion of the wafer with a probe beam and the imaging system while using up less space than a conventional X-Y stage.
One problem with using an imaging system with a R-theta stage is that as the stage is rotated, the field of view also rotates. Accordingly, it is desirable to compensate for this rotation of the field of view. A discussion of one system which provides an approach for dealing with the effect of the rotation of the field of view is discussed in the European Patent Application, EP 0 971 254, published Dec. 1, 2000; and U.S. Pat. No. 6,320,609 B1. These references are hereby incorporated herein by reference.
FIG. 1 of the ""609 patent shows a view of a prior art X Y stage, where the stage can be moved in both the X and Y direction. These prior art systems can also allow for the user to rotate the stage. By moving the stage in both the X and Y direction the user can align the center of rotation of the stage such that it is aligned with the center of the field of view of the optical imaging system. Algorithms that were used for determining the position of the field of view of the optical imaging system, or a lens of the imaging system, relative to the center of rotation of the stage took advantage of the ability to precisely control the movement of the stage in both the X and Y direction. Specifically, these prior algorithms make use of three degrees of freedom provided by a X-Y-Theta stage, and are dependent on the ability to bring any point on the stage (and a wafer mounted on the stage) to the center of the field of view after an arbitrary theta rotation.
The system described in the ""609 patent assumes that the center of rotation of the polar coordinate stage can be made nearly coincident with the center of the field of view of the imaging system. In practice, this alignment is difficult, if not impossible. When the center of the field of view of the optical imaging system is not aligned with the center of rotation of the stage in the home position, and where this offset is not accounted for, the system cannot accurately position a particular location on a wafer being inspected at the center of the field of view. Accordingly, it would be desirable to determine the exact amount of displacement of the center of rotation of the stage from the center of the field of view of the optical imaging system and correct for that offset, so that the precise location of a site on a wafer sample can be accurately controlled.
The present invention is directed to a system and a method for determining an offset between a point in the field of view of an optical imaging system of an inspection system and a center of rotation of a polar coordinate stage. The system and method take advantage of the fact that a distinctive site on a wafer being inspected can be positioned in the field of view with the polar coordinate stage being in one of two possible orientations. This is done by positioning the polar coordinate stage in a first position with a site at a particular location in the field of view, and then positioning the polar coordinate stage in a second position where the site is again positioned at the particular location in the field of view. The processor system of the inspection system is then used to calculate the offset, based on the movement of the stage necessary to change the position of the polar coordinate stage from the first position to the second position.